Embodiments of the present invention relate generally to gas turbine engines, and more particularly to turbine nozzles for such engines incorporating airfoils made of a low-ductility material.
A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine (also referred to as a gas generator turbine) includes one or more stages which extract energy from the primary gas flow. Each stage comprises a stationary turbine nozzle followed by a downstream rotor carrying turbine blades. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and should generally be minimized.
Metallic turbine structures can be replaced with materials having better high-temperature capabilities, such as ceramic matrix composites (“CMCs”). The density of CMCs is approximately one-third of that of conventional metallic superalloys used in the hot section of turbine engines, so by replacing the metallic alloy with CMC while maintaining the same part geometry, the weight of the component decreases, as well as the need for cooling air flow.
While CMC materials are useful in turbine components, they require additional design considerations when being mounted to other components as compared to their metallic counterparts. CMC materials have relatively low tensile ductility or low strain to failure when compared with metals. Also, CMCs have a coefficient of thermal expansion (“CTE”) approximately one-third that of superalloys. The allowable stress limits for CMCs are also lower than metal alloys which drives a need for simple and low stress design for CMC components.
Transferring loads out of a ceramic component is best done by distributing the load across large areas, rather than using point or line contacts. Unfortunately, rocking motion of a component such as a turbine nozzle, due to the thermal mismatch of the structural components, along with prior art pinned configurations, tends to introduce line contacts.
Prior art CMC turbine nozzles have utilized contoured contact areas to help to control the line contact as the nozzle rocked relative to the structures, and pins were replaced by adding pads to the structural hardware. However, these modifications increase the complexity and machining processes required to manufacture the structure.
Accordingly, there is a need for an apparatus for mounting CMC and other low-ductility turbine nozzles that minimizes mechanical loads on those components, with minimum complexity.